The present invention relates to an igniter system using a power IC which incorporates a vertical power semiconductor device.
FIG. 12 is a block circuit diagram of a conventional igniter system that includes an IGBT 1 (insulated-gate bipolar transistor) as a switching element; a current detection resistor 3 which is connected to a current detection emitter terminal (sense emitter terminal) of the IGBT 1; a gate resistor 4 for the IGBT 1; a current limiting circuit 31; an overheat detection circuit 32; and a self-shutoff circuit 33. The operations of the current limiting circuit 31, the overheat detection circuit 32, and the self-shutoff circuit 33 will be described later. The IGBT 1 and protection circuits such as the current limiting circuit 31, the overheat detection circuit 32, and the self-shutoff circuit 33 are formed on the same semiconductor substrate and constitute a power IC 101.
The power IC 101 is combined with an ignition coil 103 to constitute an ignition device 100 for an internal combustion engine. The ignition device 100, a combustion chamber 300 having an ignition plug 18, and an engine control unit (hereinafter referred to as ECU) including a gate drive circuit 201 for the IGBT 1 constitute an igniter system.
The ignition coil 103 is composed of a primary coil 14 which is connected to the IGBT 1, a secondary coil 15 which is connected to the ignition plug 18, and a core 16. A current flowing through the primary coil 14 is on/off-controlled by the IGBT 1.
The ECU 200 is composed of various control circuits for controlling the entire internal combustion engine system including the igniter system, and is equipped with the IGBT gate drive circuit 201 which outputs, to the power IC 101, a gate signal for on/off-controlling the IGBT 1. The ECU 200 is also equipped with a control circuit for controlling the flow of fuel or fuel gas being sent to the combustion chamber 300 from a fuel tank 400 via a valve 500. Furthermore, the ECU 200 outputs, to the power IC 101, a gate signal for turning off the IGBT 1 in response to a signal that is supplied from each of the protection circuits formed in the power IC 101.
Next, the operation of the igniter system will be described. When the IGBT 1 is turned on, a primary current starts to flow through the primary coil 14. The primary current is a current that flows through the IGBT 1, that is, a collector current of the IGBT 1. The primary current i increases with a slope di/dt=VB/Lc, where VB is a power supply voltage and Lc is the inductance of the ignition coil 103. When the primary current has flowed for a prescribed time, an off signal is supplied from the gate drive circuit 201 of the ECU 200 to the gate of the IGBT 1, whereupon the IGBT 1 is turned off. The prescribed time is set in the ECU in advance according to the engine rotation speed.
When the IGBT 1 is turned off, the energy stored in the primary coil 14 is transmitted to the secondary coil 15, whereby the voltage across the ignition plug 18 of the combustion chamber 300 is increased and the ignition plug 18 is discharged. Upon discharge, the unburned gas that has flowed into the engine (combustion chamber 300) is burned explosively with the aid of a catalyst and thereby pushes down the piston and rotates the engine. The engine rotation speed is varied by varying the frequency of reciprocation of the piston by varying the discharge frequency.
The protection circuits formed in the power IC 101 will be described below. The IGBT 1 is used as a switching element for on/off (energization/shutoff)-controlling the primary current of the ignition coil 103.
The following protection circuits for protection against overcurrent, overheat, and abnormal energization (surge current) are provided in the power IC 101 which is part of the ignition device 100 for an internal combustion engine. As for overcurrent, the current limiting circuit 31 limits the primary current of the ignition coil 103 to a preset value by controlling the gate voltage by detecting the primary current. This circuit prevents destruction due to overcurrent. As for overheating, the overheat detection circuit 32 detects the chip temperature and, if it becomes higher than a prescribed temperature, shuts out the primary current forcibly by short-circuiting the gate to the ground. This circuit prevents abnormal heating of the IGBT 1 and thereby prevents its thermal destruction. The chip temperature is detected by a diode that is formed in the chip. More specifically, the temperature dependence of the forward voltage drop of the diode is utilized. As for abnormal energization, the timer-type self-shutoff circuit 33, which is equipped with a timer for measuring the on-time of an ignition signal, shuts off the primary current forcibly by short-circuiting the gate to the ground when the ignition signal has been on for more than a prescribed time. Thin-line arrows in the figures that are associated with the current limiting circuit 31, the overheat detection circuit 32, and the self-shutoff circuit 33 indicate exchange of signals.
The use of the above protection circuits secures the necessary level of reliability of the igniter system because upon occurrence of an abnormality the corresponding protection circuit turns off the IGBT 1 and the supply of fuel (unburned gas) to the combustion chamber 300 is stopped by the valve 500 in response to an output signal of the ECU 200.
JP-A-9-42129 discloses a one-chip device for reliably detecting disconnection or short-circuiting of an ignition control signal line and for preventing re-energization during an on-period of the ignition control signal. The one-chip device is composed of an IGBT for controlling energization/shutoff of the primary current of an ignition circuit, a current limiting circuit for limiting a current flowing through the IGBT, a thermal shutoff circuit for shutting off the energization of the primary current forcibly upon occurrence of an abnormality, and a latch circuit for latching an output of the thermal shutoff circuit.
In recent years, it has come to be required to further increase the reliability of the igniter system by detecting not only the above kinds of abnormalities but also a coil failure. If a coil failure occurs, ignition may fail to cause misfires. If misfires occur, the combustion chamber 300 is filled with unburned gas and the catalyst (noble metal such as palladium or platinum) existing in the combustion chamber 300 is exposed to the unburned gas and thereby oxidized. The temperature of the catalyst increases rapidly, as a result of which the catalyst is melted or deteriorated. Once the catalyst is melted or deteriorated, ignition no longer succeeds. The reliability of the igniter system is thus lowered.
Examples of coil failures are primary coil layer short-circuiting, secondary coil layer short-circuiting, and secondary coil disconnection. The coil layer short-circuiting is a phenomenon that the coating of a coil wire that is wound in layers is damaged locally to cause contact between portions of the coil wire. If this phenomenon occurs, the inductance of the ignition coil is varied.